A conventional wireless system base station transmitter is typically configured such that the analog radio frequency (RF) path, which generally comprises RF circuitry and a power amplifier, is operating under an open-loop condition. As a result, the gain of the RF path is subject to changes caused by factors such as component variation, temperature and component aging.
One approach to addressing this gain uncertainty problem involves measuring the path gains of a sample set of transmitter elements over temperature to produce a calibration curve that is stored in every transmitter. A given transmitter then uses an on-board temperature sensor and the stored calibration curve to correct for gain drift due to temperature.
Another approach involves calibrating the gain of a given transmitter at base station set up time. However, this approach can typically only ensure that the RF path gain will be within about ±1.5 dB of a desired setting. Although certain commonly-used wireless cellular code division multiple access (CDMA) standards may require only about ±2 dB of accuracy in base station output power level, the uncertainty in RF path gain may nonetheless force the designer to “oversize” the power amplifier in order to avoid violating the out-of-band emission requirements should the gain deviate from the desired setting. Since the power amplifier is generally the single most expensive item in a CDMA base station, and its cost increases with its size, use of oversized power amplifiers significantly increases the total cost of the base station. Moreover, excessive gain variation in the RF path can seriously undermine the effectiveness of baseband predistortion techniques that are commonly utilized to offset power amplifier nonlinearity.
It is therefore apparent that a need exists for an improved approach for controlling gain variations in the RF path of a wireless system transmitter, which avoids one or more of the problems associated with the conventional approaches described above.